This invention relates to converter circuits in which an input signal is converted into one suitable for an electrical device which is operated by the input signal, and more particularly to a circuit for converting the waveform of an input signal into positive and negative full-wave rectification waveforms. Such a circuit is employed as the input circuit of a protective relay device for instance.
A conventional input converting circuit of this type is as shown in FIG. 1. In FIG. 1, reference numerals 1 through 5 designate resistors; 6 and 7, diodes; and 8 and 9, operational amplifiers which form first and second operation circuits with the circuit elements 1 through 7 mentioned above.
The operation of the circuit thus organized will now be described.
In operation, a sinusoidal voltage V.sub.1 as shown in FIG. 2a is applied through the resistor 1 to the inversion input terminal of the operational amplifier 8. When the non-inversion input terminal of the amplifier 8 is grounded, a voltage V.sub.2 is developed at the connecting point of the diode 7 connected to the output terminal of the amplifier 8 and the resistor 2 which serves as a feedback resistor. The waveform of the voltage V.sub.2 is half-wave rectified, as shown in FIG. 2b. When the voltage V.sub.1 is positive, the diode 6 connected between the input and output terminals of the operational amplifier 8 is rendered conductive, i.e., the input and output terminals are short-circuited, as a result of which the voltage V.sub.2 is at 0 volts. When the voltage V.sub.1 is positive, the diode 7 blocks the flow of current to the input terminal of the operational amplifier through the resistor 2.
The voltage V.sub.2 is applied through the resistor 3 to the inversion input terminal of the operational amplifier 9, the non-inversion input terminal of which is grounded, and the output terminal 12 of the amplifier 9 is connected through the resistor 5 to the inversion input terminal of the same and is connected through the resistors 5 and 4 to the input terminal 10. As a result, a voltage V.sub.3 which is full-wave rectified as shown in FIG. 2c is developed at the output terminal 12. Therefore, if each of the resistances of the resistors 1, 2 and 3 is represented by R, the resistance of each of the resistors 4 and 5 is represented by 2R, then
when V.sub.1 &gt;0, V.sub.3 =-V.sub.1, and PA1 when V.sub.1 &lt;0, V.sub.3 =V.sub.1 -2V.sub.2 =V.sub.1 -2V.sub.1 =-V.sub.1
If a capacitor is connected in parallel with the resistor 5, which is the feed-back resistor of the operational amplifier 9, then a voltage, which is obtained by smoothing the voltage V.sub.3, is provided at the output terminal 12.
The conventional input converting circuit is designed as described above. Therefore, in order to obtain full-wave rectification and waveform smoothing, the respective circuits described above must be provided.